Cryptographic communications have been widely used to date in order to prevent information leakage and to enhance security performance. Technology whereby encryption keys employed in such cryptographic communications are generated on an authentication server, which is separate from user terminals (nodes), and distributed to each terminal is known (reference is made, for example, to Patent Document 1). Technology whereby user terminals are registered on a content server, which runs the services that provide the content, is also known (reference is made, for example, to Patent Document 2).